Current-limiting fuse



Jan. 5, 1954 F. .1. KOZ ACKA CURRENTLIMITING FUSE 3 Sheets-Sheet 1 Filed May 16, 1950 [VIII/[III] INVENTOR.

Jan. 5, 1954 KOZACKA 2,665,348

CURRENT-LIMITING FUSE Filed May 16, 1950 6 l 1 If I 3 Sheets-Sheet 2 IN V EN TOR. j

Jan. 5, 1954 F. J. KOZACKA 2,665,348

CURRENT-LIMITING FUSE Filed May 16, 1950 3 Sheets-Sheet 3 3g A peres T52 590/1mper -S 52 Raf/o [lle Crux-.Skc/n r Ileduced Crass Sec 770 )1 INVENTOR. W I

Patented Jan. 5, 1954 CURREN T-LIMITIN G FUSE Frederick J. Kozacka,

Amesbury, Mass., assignor to The Chase-Shawmut Company, Newburyport, Mass., a corporation of Massachusetts Application May 16, 1950, Serial No. 162,323

39 Claims.

This invention relates to improvements in fuses. It is more particularly concerned with current-limiting fuses, i. e. fuses which, when subjected to an A.-C. fault current of short-circuit current proportions, effect interruption of the circuit in less, or even substantially less, than half a cycle of the current wave. Such fuses, when subjected to a D.-C short-circuit current having a similar rate of rise as an A.-C. shortcircuit current, effect interruption of the D.-C. current in a similarly short period of time as in the case of A.-C. current interruption.

The invention is particularly applicable to low voltage current-limiting high current interrupting capacity fuses which may have voltage ratings up to 1200 volts.

Current-limiting fuses are called to interrupt fault currents of short-circuit current propor tions before the currents reach the maximum short-circuit current strength which the system or circuit with which the fuses are associated is capable of producing. That current is generally referred to as the available short-circuit current. The maximum current which is allowed to flow through a circuit which is protected by a current-limiting fuse is called the let-through current. Obviously the let-through current is much smaller than the available short-circuit current. Any available current exceeding the let-through current is considered a short-circuit current.

Current-limiting fuses, in addition to protecting a circuit against fault currents of short-circuit proportions, often are required also to interrupt overload currents of much smaller proportions than short-circuit currents if such overload currents are of relatively long duration. Small overload currents which a current-limiting fuse may be required to interrupt may be in the order of eight to ten times the rated current of the fuse.

A general object of the invention is to provide a fuse, more particularly a fuse of the current limiting type, which has a far greater interrupting eliiciency than any fuses previously known.

Another object of the invention is to provide a fuse, more particularly a fuse of the current limiting type, which is of much smaller dimensions or much smaller bulk than any previously known fuses of the same voltage and interrupting capacity rating.

Still another object of this invention is to provide a current-limiting use in which the are energy is far less than in any prior art currentlimiting fuse.

The small dimensions of fuses according to this invention are due'to the drastic reduction of the are energy, enabling a drastic reduction of the mass of cooling and deionizing means required for deenergizing the products of arcing formed upon blowing of the fuse. The small dimensions of fuses according to this invention are also due to their high interrupting efficiency which in turns tends to reduce the arcing time and the are energy.

Another object of the invention is to reduce the ratio of the mass of vaporizable metal within the fuse to the current-carrying capacity of the fuse to far smaller values than was ever held possible heretofore among designers of fuses. Because of the immense reduction of metal vapors, reignition after the first current zero following arc initiation can safely be precluded with a minimum of pulverulent deionizing filler material.

Quartz sand ranges among the most effective pulverulent arc quenching materials which are being used in current-limiting fuses. When subjected to the heat of the arc, quartz sand fuses into a glass-like body. The fusing process absorbs large amounts of heat suflicient to enable to achieve the afore-mentioned current-limiting action so important for high capacity fuses. The glass-like body resulting from the fusion of quartz sand is a semi-conductor of electricity which turns rapidly into an insulator if caused to cool down at a rapid rate. If the rate of cooling of said semi-conductor is not sufiiciently rapid and the semi-conductor interconnects the terminals of the fuse, a small follow-current will be permitted to flow through the fuse after initial interruption of the circuit by the fuse has been achieved. Such a follow-current may be responsible for suificient reheating of the semiconductor to cause re-kindling of the are.

It is, therefore, a general object of the invention to provide a fuse, and more particularly a current-limiting fuse, which precludes the flow of of any, or any appreciable, follow-current after initial arc'extinction and circuit interruption has been achieved.

A special object of this invention is to provide a fuse wherein the glass-like semi-conductor formed by the fusion of the pulverulent arc quenching, deionizing and cooling filler within the fuse is separated by dielectrically sound material from any live part of the fuse.

In fuses of conventional design, the pulverulent arc extinguishing filler which, for instance, may be quartz sand, is heated during the prefusion period. If the pre-fusion period is long, as in the case of protracted overloads, there is suflicient time left for a thorough heat exchange between the hot fuse link and the pulverulent arc extinguishing filler. Consequently, the pulverulent arc extinguishing filler will have reached a relatively high temperature at the time of fusion of the link and initiation of the are. In that heated condition the energy absorbing ability of the pulverulent filler is greatly impaired.

It is, therefore, a further object of the invention to provide a fuse having an effective thermal insulation between the pulverulent filler and the link, and wherein the heat flow is diverted away from the filler, and wherein particularly those portions of the filler upon which the heaviest cooling duty rests during arc extinction are thermally well insulated from the link.

In any fuse a certain amount of pressure is generated during the arcing time. The amount of that pressure depends upon the are energy which is released within the fuse and upon other factors. It has been found that are interruption is more effective if pressure is built-up to a predetermined high pressure level compatible with the mechanical strength of the structure wherein the arc burns, and if the pressure is then suddenly released, resulting in a high negative rate of change of pressure It is, therefore, a further object of the invention to provide a fuse of relatively simple design wherein the pressure generated at the arcing zone is allowed to rapidly increase up to a predetermined amount and is then suddenly released in a manner compatible with the most stringent safety requirements.

In prior current-limiting fuses the let-through current increases as the current-carrying capacity of the fuse increases. Circuits wherein the currents are large should be interrupted at much lower instantaneous values than the peak of the available short-circuit current in order to protect from detrimental thermal and electromagnetic effects any device which is connected into the circuit. In other words, there is a need for current-limiting fuses which combine high current-carrying capacity with the ability to limit the let-through current to very low values.

It is, therefore, a further object of this invention to provide a current-limiting fuse which has a much higher ratio of rated current to letthrough current than any prior art currentlimiting fuse.

A further object of this invention is to provide a fuse which is predicated on the novel limiting width concept, which concept will below be explained in detail.

Frequently fuses are required to effectively interrupt currents of short-circuit current proportions as well as relatively small protracted overload currents. As far as interruption of currents of short-circuit current proportions is concerned, silver is indicated as link metal because silver vapors have a relatively low conductivity at relatively high temperatures. Since silver has a high fusing point, use of links solely consisting of silver results during protracted overloads in an intense pre-heating of the pulverulent arc quenching filler which is used in most fuses. As mentioned before, such pre-heating deprives the filler largely of its arc quenching capacity. To avoid this, it has become accepted practice to use composite fuse links including a portion suitable for interrupting currents of short-circuit current proportions and another portion consisting of a low fusing point metal which initiates the interrupting process in case of small protracted overload currents. Such composite fuse links are, however, expensive to manufacture and subject to aging resulting, while in service,

in a change of their time current characteristics.

It is, therefore, a further object of this invention to provide a fuse having a fuse link consisting of but one metal, as distinguished from the above referred to composite or multi-metal links, in which the arc quenching potency of the pulverulent filler is not impaired by small protracted overloads.

The foregoing and other general and special objects and advantages of the invention will more clearly appear from the ensuing particular description of the invention, as illustrated in the accompanying drawings wherein Fig. 1 is a side elevation of a current-limiting fuse embodying features of the invention, a portion of the casing and portions of interior elements being broken away;

Fig. 2 is a cross-sectional view on line of Fig. 1;

Fig. 3 is a cross-sectional view on line 3-3 of Fig. 2;

Fig. 4 shows in perspective the fuse link, its confining insulating plate members, and charpreventing sheets, in separated relation;

Fig. 5 is plan view of the fuse link with a heatdistribution pattern diagrammatically represented thereon by stippling;

Fig. 6 is a medial cross-sectional view of a current-limiting fuse embodying features of the invention and having a plurality of fuse links in series;

Fig. 7 is a perspective view of a cylindrical form of fuse link embodying features of the invention;

Fig. 8 is a medial cross-sectional view of a fuse equipped with a fuse link of the cylindrical form of Fig. 7; I

Fig. 9 is a cross-sectional view on line 9-9 of Fig. 8;

Figs. l017 are plan views of fuse links having equal lengths, equal thickness, and uniform fusing neck dimensions, but having progressively increased widths, and each having heat patterns diagrammatically represented thereon by stippling; and

Fig. 18 is a diagram illustrating the phenomenon of reduction of the rate of increase of the current-carrying capacity of a fuse link with increase of the width of the link.

Referring to the drawings, the invention is illustrated as it may be embodied in a current-limiting fuse having a pair of terminal elements l0, 12 which include the plug portions l4, [6 for nicely fitting within and closing the opposite ends of a tubular insulating casing member l8 which may be secured to the plug portions [4, I6 in any suitable manner, as by screws 20. The opposed inner faces of plug portions l4, [6 have the in-. wardly extending terminal projections 22, 24 thereon, and a fuse link 26 extends within casing member l8 interconnecting the terminal projections 22, 24. Conveniently, the terminal projections may be notched or slit at 28 for reception of the opposite end portions of the fuse link 26, in which case the parts of the terminal pro-- jecting on opposite sides of each notch or slit 28 may be drawn toward each other by screws 30 to clamp the end portions of the fuse link.

In Figs. 1-6 and 10-17, the fuse link 26'is a thin flat strip of a suitable fuse link material whose length need never substantially exceed two inches. However, it is a feature of importance that the link is designed to have width approximating th maximum width beyond which any;

more increase in width would have no appreciable eifect upon the current-carrying capacity of the link The fuse link 26, as herein illustrated, has a single zone 32 of reduced cross-section at which two extremely small and short reduced cross-section elements 34 are in parallel between the two relatively much longer larger cross-section portions 36. The width of the fuse link preferably is determined by the ability or capacity of the larger cross-section portions 36 to absorb heat from the reduced cross-section elements 34 when the fuse link is carrying current of load proportions or subjected to a moderate overload of relatively long duration. Under such conditions the temperature of the reduced cross-section elements 34 of the link rises very slowly because the heat generated at the reduced cross-section elements is absorbed at a more rapid rate by the larger cross-section portions until finally a, con dition is reached in which the larger cross-section portions are incapable of absorbing heat as fast as heat is generated at the reduced crosssection elements 34. Then, the temperature of the latter rises until they may attain their melting temperature. The space between th two necks 34 is about twice the space between each neck and the adjacent edge of the link. The necks 34 are SllfilCiBIltly short and of sufficiently small width to cause a system of virtually circular isothermes to form on the surface of the link. If the link is made of copper, the heat distribution pattern becomes readily visible on account of different discoloration of copper when subjected to different temperatures. In Fig. 5 the heat distribution pattern of a link according to the invention has been indicated more or less diagrammatically by stippling'. Each neck 34 is associated with an immediately adjacent region 38 in which the isothermes may be considered to be circular. This region is represented by heavier stippling. It is obviously the high temperature region of the link 26. The outer lower temperature region of the link 26 is generally of oval shape and represented by lighter stippling.

The thickness of the link 26 may be of the order of .010 inch although the thickness may be varied considerably in links of different ratings.

Generally, the dimensions of the reduced crosssection elements or necks 34 will be so small and the larger cross-section portions 35 relatively so much larger that the links are likely to become ruptured in handling unless care is taken to avoid rupture.

According to the invention, the fuse links are amply supported and protected against accidental rupture when mounted in the casing l 8, by being clamped and confined between the rigid insulating plate members 44 which may besecured on opposite sides of the fuse link 26 by the same screws 33 which secure the end portions of the link to the terminal projections 22, 24. The plate members 44 are made of an organic insulatin material which has a suiliciently high mechanical strength, e. g. vulcanized fiber. Such insulating materials evo' 1e gases when subjected to the heat of the arc and the gases so evolved may be utilized for cooling, diluting and deionizing the arc stream. Preferably, thin sheets 45, of mica or the like, intervene between the adjacent surfaces of the link and insulating plate members 44, to avoid charring of the insulating plate members when the fuse link becomes heated. These mica sheets 46, or the like, are

provided with holes 4'! opposite the reduced cross-section regions 34 of the fuse link. The insulating plate members 44 are provided with holes 50 arranged in registry with the holes 41 and the neck portions 34.

Suitable arc-extinguishing pulverulent material 48 preferably surrounds the insulating plate members 44 Within the casing I 8. The pulverulent material 48 may consist of clean quartz sand which is characterized by its high heat absorbing capacity and by the fact that it evolves virtually no gas when subjected to the heat of the are. This latter property of quartz sand is de sirable because it helps in keeping the pressure inside the fuse within the limits dictated by the mechanical strength of the casing. A potential danger generally inherent in the use of fuses of quartz sand as deionizing and cooling medium consists in that fused quartz sand forms a glasslike substance which is semi-conductor as long as it is at a high temperature. As will be shown. this potential danger has been completely eliminated in fuses according to the present inventlon.

It is a feature of importance that the fuse link 26 is substantially completely confined and isolated by the insulating plates 44 between which the fuse link is sandwiched, with the pulverulent arc-extinguishing material 48 maintained out of contact with the fuse link excepting along the thin side edges of the fuse link, although, if desired, the edges of the link may be covered to completely isolate the fuse link from the material 48. Hence, the pulverulent arc-extinguishmaterial remains relatively cool when the fuse becomes heated to high temperatures.

On the occurrence of a fault condition in the circuit which is protected by the fuse, fusion is initiated at the two narrow neck portions 34 which constitute parallel current paths. Therefore two arcs are initiated in parallel. Because two arcs in parallel are unstable, one of the arcs will tend to be rapidly extinguished. Arc extinction is eifected by blasts of are products out of the high pressure zone formed between the two insulating plates 44 through the venting holes or expulsion ports 41 and 52 formed in the mica sheets 45 and the heavier organic insulating plates 44. The hot are products escaping through these holes are instantly cooled and deionized as they come into contact with the cold quartz sand 48 arranged within the casing l8 on the outside of the plates 44. Fusion of the quartz 48 will occur adjacent the downstream ends of the expulsion openings 41, 52; but the lumps of fused sand thus formed will not extend all the way between the terminals of the fuse and will be formed at a point other than that previously occupied by the link. As an additional safety measure against bridging of the terminal elements l4 and 16 by a hot quartz mass of relatively low resistance, the terminal elements l4 and 16 are covered at their juxtaposed sides by disc-shaped insulating barriers 54.

Since it has been found that are extinction can be achieved more effectively if the pressure is althen suddenly released, the fuses according to the present invention are provided with a simple means for achieving such a desirable pressure time characteristic. To this end, each of the holes 56 is provided with a valve means for substantially closing the hole until after arc-generated pressure within the space between the plates 44, and particularly at the inner end of the hole, exceeds a predetermined level. Such a valve means may be constituted by sealing means 52 in the shape of small discs responsive to heat and pressure for obstructing each of the holes until heat and pressure caused by vaporization of the necks 34 and the adjacent portions of the link 26 exceed predetermined amounts. Obviously, each of the two holes 58 in each of the plates it forms a distinct venting passage, and venting is efiected in opposite directions through each of the plates M. The sealing discs 52 are arranged at the downstream ends of the ventin holes 59 and preclude any direct contact between the granular filler d8 within the casing l8 and the link 25. Therefore, the heat generated at the necks 34 will not be transmitted directly to the filler 38 at the downstream ends of holes 50, the hole spaces constituting relatively good thermal insulators diverting the main heat fiow originating. at the necks away from the portions of the filler 48 adjacent holes 53 and causing the heat to flow mainly axially along the link 26. Because of this pattern of the heat flow, the hot arc products escaping thorugh the holes 50 impinge, upon leaving the holes 58, upon a relatively cool pulverulent body, which greatly enchances the latters effectiveness.

In some cases the pulverulent body 48 may be omitted, though its presence increases considerably the interrupting capacity of the structure.

The cross-section of the holes 50 should be small enough to preclude destruction of the link-sandwiching plates M by arc-generated pressure, and should be large enough to cause a sudden breakdown of pressure upon destruction of discs 52.

The fuse link 26 is relatively thin, i. e. in the nature of a metal foil and the cross-section of the necks is extremely small. Sheet metals within the thickness range of .005 to .025 inches proved to be most satisfactory.

No prior art fuses of the same current rating have ever been provided with links having a portion of reduced cross-section of so small a crosssectional area as contemplated by the present invention. The drastic reduction of the cross-sectional areas of the necks has been made possible in the fuses according to the invention by reducing the neck length so as to convert the neck virtually to a point heat source, and by drastically increasing the heat exchange area of the large cross-section portions of the fuse link situated on both side of the necks 34. While the above described configuration of the fuse link 26 results in an increase of its current-carrying capacity due to intense cooling of the necks 34, the smallness of the cross-sectional area of the neck 34 results in a drastic reduction of the let-through current. This is due to the fact that the rapid rate of rise of short-circuit currents and the rapid rate of generation of heat by short-circuit currents preclude the occurrence of such heat exchange phenomena as determine the behaviour of the fuse under small protracted overload conditions.

While fuse links with one neck or two necks in parallel have been known heretofore, the concept of a superwide fuse link with a supershort and. supernarrow neck, combining high currentcarrying ability and drastic reduction of the letthrough current and also drastic reduction of are energy is entirely new in the art. It contradicts all previous views relative to the design of ourrent-limiting fuses which are to the effect that the let-through current must increase in a certain way with increase-0f the current-carrying capacity of thefuse. H y I f It will be observed from Fig. 3 that the width of the organic insulating barriers M is only slightly less than the internal diameter of the casing 18. Thus the chamber formed within casing 18 is being subdivided in a direction substantially longitudinally thereof into two substantially separated subchambers. The filler 48 in both these subchambers is being utilized to cool and deionize the hot products of arcing. Since each subchamber is fed with hot arc'products through two spaced venting holes, the thermal duty imposed upon the total mass of pulverulent filler enclosed in both subchambers is fairly evenly distributed. The are products flowing through the holes 52 are sufficiently hot to cause evolution of some gas from the walls which laterally bound the holes. The gas so evolved dilutes and cools the arc products and tends to increase their dielectric strength by turbulently mixing with them. Considerable blast velocities may be reached within the holes 50. Since the area of the surface at which an interaction between the arc and organic matter takes place is extremely small, and since the time of interaction is extremely short, the evolution of gas from the organic insulating plates 44 is sufficiently small to remain within the desirable and required limits.

It may be observed that the stopping of the insulating barriers 44 short of actual contact with the cylindrical wall of the casing 18 in Fig. 3 permits filler material 4.8 to accumulate in relatively small amounts along the side edges of the in- 'sulating plates and in direct contact with the side edges of the fuse link 26. This filler material has access to the notched zone 32 of the fuse link, across which the necks 34 extend. Actually, however, no appreciable amount of the filler enters the zone 32, between the insulating plates because of the small dimensions of the zone and, under pressure conditions, the escape of products of a fusing of necks 34 laterally through zone 32 is insufiicient to deleteriously affect the operation of the device as previously described. However, if considered desirable or necessary, the outer ends of the zone passage 32 may be plugged or otherwise closed against entrance of filler material. For example, the insulating plates 44 may have press fit in the casing so that no filler material 48 can get to the side edges of the plates or the link.

. .When it is desirable to provide fuses embodying the features of the invention which have higher voltage ratings than are attainable with the devices of the Figs. 1-3 form, they may embody a series arrangement of the Figs. 1-3 structure, as illustrated in Fig. 6. In the Fig. 6 embodiment, two fuse links 26 are mounted in separate chambers within an insulating casing member l8, on opposite sides of a central partition member l5 which also is a conducting member connecting the inner ends of the two fuse links. Each fuse link 26 is closely confined between insulating plate mem-, bers 44 as in the Figs. 1-3 form, preferably with pulverulent arc-extinguishing material 48 between the plate members and the casing member I8. The circuit-interrupting and currentlimiting action of the fuse of Fig. 6 is substantially the same as in the Figs. 1-3 form.

Fig. 7 illustrates a cylindrical type of fuse link 26' which embodies features of the invention, it having a single zone 32' of restricted cross-section intervening between relatively much larger cross-section portions 36. Figs. 8 and 9 show this cylindrical type of fuse link embodied in a fuse having generally the same structural and operational features described in connection with the Figs. 1 -3 embodiment. However, instead of flat insulating plates for confining the fuse link, the Figs. 8 and 9 embodiment has an inner plug member 45 within the fuse link and a generally cylindrical insulating member 45' encircling the fuse link exteriorly thereof, with pulverulent arcextinguishing material d8 between the insulating member 45' and an insulating casing member [8. Fuse link 26' has two restricted cross-section conducting elements 34 at its restricted cross-section zone 32', and the cylindrical insulating member 45' has a hole 58 therethrough opposite each restricted cross-section element 34. The operation of the Figs. 8 and 9 fuse is similar to the operation of the fuse of Figs. 1-3.

Fuse links 26 and 26' as herein described may be made of copper, zinc, silver, or any other suitable fuse link material. Whatever the material, the restricted cross-section conducting elements .34 or 34' at the single restricted cross-section zone 32 or 32' will be extremely small, and the adjacent portions 36 or 36' will have substantially greater width than prior comparable fuse In Fig. reference numeral 49 has been applied to the lateral edge of the link 26. The reference numeral 42 indicates the lateral edge of a fuse link having generally the same geometry as link 26, but having a larger width. The outlin of such a link of larger width has been indicated by dotted lines. Obviously, a fuse link could be conceived having generally the same geometry as link 26, but having a smaller width. It is important to determine the optimum width, or the range of optimum width. How to determine the optimum width or the range of optimum width will now be set forth in connection with Figs. 10 to 7, nclusive.

These figures represent eight different fuse links having equal length and equal thickness and each having one restricted cross-section portion 32 formed by two identical necks 34. The cross-sectional area of all the necks is the same. The width of the link increases progressively from the link of Fig. 10 to the link of Fig. 17. The minimum fusing current of each link, or the maximum current which each link is capable of carrying without fusing, increases in proportion to the width of the link up to the critical width shown in Fig. 15. A further increase of the width of the link does not result in a further increase of the current-carrying ability of the link. The links shown in Figs. 16 and 17 have a larger width than the link of critical width shown in Fig. 15, but their current-carrying ability is not larger than the current-carrying ability of the link of Fig. 15. The maximum current which any of the links shown is able to carry indefinitely has been indicated in Figs. 10 to 17. In Fig. 18 that current has been plotted against the ratio of the large cross-section portion 36 to the reduced cross-section portion 32 of the link, i. e. the portion which consists of the two necks 34. It appears from Fig. 18 that a progressive increase of the above ratio results in a progressive increase of the current-carrying ability of the link. In the particular series of tests to which Fig. 18 refers a. critical ratio is reached at about 45:1, i. e. a further increase of the width of the link does not result in the increase of its current-carrying capacity.

- In order to arrive at a link which combines a high current carrying capacity with low letthrough current, the ratio of the cross-section of the large cross-section portion of the link to the cross-section of the small cross-section poretion of the link should preferably lie within the range of ISA-50:1. For the particular kind of links to which Fig. 18 refers the optimum range for the above ratio lies between 30:1 and 45:1, and the latter ratio may be deemed to be the best from the viewpoint of reconciling high current-carrying capacity and low let-through current.

Figs. 10 to 18, inclusive, refer to links of .010 inch copper and Figs. 10 to 17, inclusive, are drawn true to scale. In the latter figures the heat pattern resulting from discoloration of the copper has been indicated by stippling. It appears from the heat pattern indicated by stippling that the necks 34 are so short and of so small a width that they can be considered point heat sources. Consequentl a system of virtually circular isothermes is formed on the links. Reference numeral 38 indicates th hottest area of the links and reference numeral 39 has been applied to indicate a relatively cooler area of the surface of the link.

carrying ability, the longitudinal edges are at ambient temperature.

The new concept that by increasing the width present invention are, at least in part, predicated upon the limiting width concept.

It will be understood that I have illustrated and described herein preferred embodiments only of the invention and that various alterations may be made in the details thereof without departing from the spirit and scope of the invention as defined in the appended claims.

I claim as my invention:

1. In a fuse the combination of a fuse link having a zone of restricted cross-section, insulating means in immediate contact with and enclosing said link and defining an orifice for per mitting the escape of metal vapors formed upon vaporization of said zone of restricted cross-section, said orifice extending generally in a direction transversely of said link, a pulverulent substance on the outside of said insulating means for receiving said metal vapors escaping from said orifice, and a casing of insulating material enclosing said pulverulent substance.

-2. In a fuse the combination of a fuse link having a zone of restricted cross-section, a pair of insulating plates for sandwiching said link therebetween, at least one of said pair of plates defining at least one orifice for permitting the escape of metal vapors formed upon vaporization of said zone of restricted cross-section, said orifice being arranged substantially in registry with said zone of restricted cross-section and extending generally in a direction transversely of said link, a body of quartz sand on the outside of said pair of insulating plates for receiving said metal vapors escaping from said orifice, and a casing of insulating material enclosing said link, said pair of insulating plates and said body of quartz sand.

3. In a fuse the combination of a ribbon-type fuse link having a zone of restricted cross-section, a pulverulent substance for de-ionizing the are products formed upon vaporization of said restricted cross-section zone, and insulating barrier means for separating said link from said pulverulent substance, said barrier means defining an expulsion port of narrow cross-section arranged in a direction transversally of the current flow in said link for producing a turbulent blast of metal vapors into said pulverulent substance upon vaporization of said restricted cross-section zone.

4. In a fuse the combination of a ribbon-type fuse link having a zone of restricted cross-section, a pulverulent inorganic substance surrounding said link for de-ionizing the are products formed upon vaporization of said restricted crosssection zone, a pair of insulating plates of an organic substance for sandwiching said link therebetween and separating said link from said inorganic substance, said pair of insulating plates defining a pair of expulsion ports arranged in a direction transversally of the current fiow in said link and extending in opposite directions for permitting metal vapors to escape simultaneously in opposite directions into said inorganic substance upon vaporization of said restricted crosssection zone.

5. In a fuse the combination of a fuse link having a zone of restricted cross-section, insulating means disposed at opposite sides of and closely confinin said fuse link and defining an orifice for permitting escape of metal vapors formed upon vaporization of said zone of restricted cross-section, said orifice extending generally in a direction tranversely of said link, enclosure means defining a chamber outside of said insulating means for receiving said metal vapors escaping from said orifice, and a body of quartz sand within said chamber.

6. In a fuse the combination of a fuse link having a zone of restricted cross-section, two solid insulating members disposed at opposite sides of and closely confining said fuse link therebetween, at least one of said members defining at least one orifice for permitting the escape of metal vapors formed upon vaporization of said zone of restricted cross-section of said fuse link, said orifice being arranged substantially in registry with said zone of restricted cross-section and extending generally in a direction transversely of said link, enclosure means defining a chamber outside of said insulating members for receiving said metal vapors escaping from said orifice, and a body of quartz sand within said chamber.

I 7. In a fuse the combination of a ribbon-type fuse link having a zone of restricted cross-section, enclosure means for said fuse link defining a chamber around said fuse link, insulating barrier means closel confining said fuse link and maintaining said fuse link substantially out of contact with contents of said chamber, said barrier means defining an expulsion port of narrow crosssection arranged in a direction transversally of the current flow in said fuse link for producing a turbulent blast of metal vapors into said chamber upon vaporization of said zone of restricted cross-section of said fuse link.

8. In a fuse the combination of a ribbon-type fuse link having a zone of restricted cross-section, enclosure means for said fuse link defining a chamber around said fuse link, a pulverulent fulgurite-forming arc-quenching filler within said chamber, solid insulating members closely .con-

fining said fuse link therebetween and isolating said fuse link from said arc-quenching filler within said chamber, and means defining a pair of expulsion ports arranged transversely to the general direction of current fiow in said link and extending in generally opposite directions for permitting metal vapors to escape simultaneously in opposite directions from between said solid in sulating members into said arc-quenching filler within said chamber upon vaporization of said zone of restricted cross-section of said fuse link.

9. In a fuse the combination of a fuse link having a zone of restricted cross-section, solid insulating means in immediate contact with and enclosing said link and defining an orifice for permitting the escape of metal vapors formed upon vaporization of said restricted cross-section zone, a pulverulent substance at the exhaust end of said orifice for receiving the metal vapors escaping from said orifice, and valve means for substantially closing said orifice until after'arcgenerated pressure adjacent said restricted crosssection zone exceeds a predetermined amount of pressure.

10. In a fuse the combination of a fuse link having a zone of restricted cross-section, a. pair of insulating plates for sandwiching said link therebetween and defining at least one orifice for permitting the escape of metal vapors formed upon vaporization of said restricted cross-section zone, said orifice being arranged substantially in registry with said zone of restricted cross-section and extending generally in a direction transversely of said link, a body of pulverulent inorganic material on the outside of said pair of in sulating plates for receiving said metal vapors escaping from said orifice, and sealing means responsive to heat and pressure for obstructing said orifice until heat and pressure caused by vaporization of said restricted cross-section zone have exceeded predetermined values.

11. In a fuse the combination of a fuse link having a zone of restricted cross-section, solid insulating means closely confining said fuse link and defining an orifice for permitting %cape of metal vapors formed upon vaporization of said restricted cross-section zone, enclosure means defining a chamber outside of said solid insulating means and opposite the exhaust end of said orifice for receiving the metal vapors escaping from said orifice, and valve means for substantially closing said orifice until after arc-generated pressure adjacent said restricted cross-section zone of the fuse link exceeds a predetermined amount of pressure.

12. In a fuse the combination of a-fuse link consisting of a strip of sheet metal of relatively considerable width, said link having a neck sufficiently short and sufiiciently narrow to result in the formation of a system of virtually circular isothermes on the surface of said link, insulating means in immediate contact with and enclosing said link and defining an orifice permitting the escape of metal vapors formed upon vaporization of said neck, said orifice extending generally in a direction transversely of said link, and a pulverulent substance of high heat absorb ing capacity on the outside of said insulating means for receiving said metal vapors escaping from said orifice.

13. In a fuse the combination of a substan-' tially strip-shaped fuse link of relatively consid' erable width, said link having a pair of necks forming parallel current paths of highly'restricted cross-section, the spacing of said pair-of-necks from each other by far exceeding the spacing of each of said pair of necks from the adjacent outer edge of said link, insulating means in immediate contact with and enclosing said link and defining a pair of orifices for permitting the escape or" metal vapors formed upon vaporization of said pair of necks, each of said pair of orifices being associated withone of said pair of necks and extending generally in a direction transversely of said link, and a pulverulent substance of high heat absorbing capacity for receiving said metal vapors escaping from said pair or" orifices.

14. In a fuse the combination of a substantially strip-shaped fuse link of relatively considerable Width, said link having a pair of necks forming extremely short parallel current paths or highly restricted cross-section, the spacing of said pair of necks from each other by far exceeding the spacing of each of said pair of necks from the adjacent outer edge of said link and the spacing of each of said pair of necks from the adjacent outer edge of said link being of the order required to cause the outer edges of said link to remain substantially at ambient temperature when said link is subjected for an extended period of time to the highest current which it is capable of carrying indefinitely, insulating means in immediate contact with and enclosing said link and defining a pair of orifices for permitting the escape of metal vapors formed upon vapor of said pair of necks, each said pair of or s being associated With one of said pair of and extending generally in a direction transversely of said link, and a pulverulent non-gasevolving substance of mineral origin 11 heat absorbing capacity for receiving said metal vapors escaping from said pair of orifices.

15. In a fuse the combination or" a in consisting of a strip of metal foil, said link portions of relatively considerable crose-sec and relatively considerable Width and an int mediate portion of very restricted cross-section and very short length defining a point heat source, the ratio of the cross-section of said considerable cross-section portions to the cross-section of said intermediate portion being at least fifteen to one, insulating means in immediate contact with and enclosing said link defining an orifice extending generally in direction transversally of the current flow in said link and permitting the escape of metai vapors formed up on vaporization of said intermediate portion, and a pulverulent su stance of high heat absorbing capacity on the outside of insulating means for receiving said metal vapors escaping from said orifice.

16. In a fuse the con ination of fuse link having portions of relatively considerable crosssection and or relatively considerable width and an intermediate portion of restricted crossection and restricted length defining a point heat source, the ratio of the cross-section of said con siderable cross-section portions to the crosssection of said intermediate portion exceeding thirty to one, a pulverulent substance for deionizing the are products formed upon vaporization of said intermediate portion, and barrier means of insulating material for said intermediate portion from said pulverulent substance, said barrier means defining at least one expulsion port of narrow cross-section extending in a direc ion generally transversally of the curr flow in said link, for producing a turbulent blast of metal vapors into said pulverulent substance upon vaporization of said intermediate portion.

17. In a fuse the combination of a fuse link having portions of relatively considerable crosssection and a pair of intermediate neck portions or" restricted cross-section defining parallel current paths, the ratio of the cross-section of said considerable cross-section portions to the total cross-section of said pair of neck portions being at least thirty to one, a pulverulent substance for deionizing the are products formed upon vaporization of said pair of neck portions, and barrier means of insulating material for separating said pair of neck portion from said pulverulent substance, said barrier means defining at least one pair of expulsion ports of restricted crosssection, one associated with each of said pair of neck portions, for producing a pair of turbulent blasts of metal vapor from said neck portions into said pulverulent substance upon vaporization of said pair of neck portions.

18, In a current-limiting fuse the combination of a substantially strip-shaped fuse link having portions of relatively considerable cross-section and a pair of intermediate neck portions of restricted cross-section defining parallel current paths, the spacing of said neck portions from each other by far exceeding the spacing of each of said pair of neck portions from the adjacent outer edge of said link, the ratio of the cross-section of said considerable cross-section portions to the total cross-section of said pair of neck portions being at least thirty to one, a pulverulent substance for deionizing the are products formed upon vaporization of said pair of neck portions, and barrier means of an organic insulating mate l for separating said pair of neck portions frosaid pulverulent substance, said barrier means defining at least one pair of expulsion ports of relatively narrow cross-section, one associated with each of said pair of neck portions, for producing a pair of blasts of metal vapor from the zones of said pair of neck portions into said pulverulent substance upon vaporization of said pair of neck portions.

19. In a current-limiting fuse the combination or fuse link means including a plurality of large cross-section portions and a plurality of restricted cross-section portions of extremely short length, the current paths defined by each of said plurality of large cross-section portions being serially related to the ourent paths defined by each or" said plurality of restricted cross-section portions, a pulverulent substanc for deionizing the are products formed upon vaporization of said plurality of restricted cross-section portions, and axially aligned barrier means for separating said plurality of restricted cross-section portions from said pulverulent substance, said barrier means defining a series of relatively narrow spaced transverse vents, at least one associated with each of said plurality of restricted cross-section portions, for venting into said pulverulent substance metal vapors formed by said plurality of restricted cross-section portions upon vaporization thereof.

20. In a fuse the combination of substantially strip-shaped fuse link means including a plurality of large cross-section portions and a plurality of restricted cross-section portions of extremely short length, the current paths defined by each of said plurality of larg cross-section portion being serially related to the current paths defined by each of said plurality of restricted cross-section portions, each of said plurality of restricted cross-section portions consisting of a pair of spaced neck portions defining parallel current paths, the spacing of said pair of neck portions from one another by far exceeding th spacing of each of said pair of neck portions from the adjacent outer edge of said link, the ratio of the cross-section of each of said plurality of large cross-section portions to the total cross-section of each of said pair of neck portions being at least thirty to one, a pulverulent fusible substance of high heat absorbing capacity surrounding said plurality of restricted cross-section portions, and aligned barrier means of insulating material for separating said plurality of restricted cross-section portions from said pulverulent substance, said barrier means defining a group of relatively narrow spaced vents, at least one pair associated with each pair of said neck portions, for venting into said pulverulent fusible substance the metal vaports formed at each said pair of neck portions upon vaporization thereof.

21. In a current-limiting fuse the combination of a fuse link having portions of relatively large cross-section and an intermediate portion of restricted cross-section and restricted length, the ratio of the cross-section of said relatively large cross-section portions to the cross-section of said intermediate cross-section portion being at least thirty to one, a pulverulent inorganic substance surrounding said intermediate portion for deionizing the are products formed upon vaporization thereof, a pair of insulating plates for sandwiching said link therebetween and separating said link from said substance, said pair of insulating plates defining expulsion ports arranged in a direction generally transversally of the current flow in said link and extending in opposite directions for permitting metal vapors to escape into said substance simultaneously in opposite directions upon vaporization of said intermediate portion of said link.

22. In a current-limiting fuse the combination of a tubular casing, a pair of terminal elements associated with the ends of said casing, a fuse link interconnecting said terminal element, said link having portions of relatively large cross-section and an intermediate portion of restricted crosssection consisting of a pair of spaced necks defining parallel current paths, the ratio of the cross-section of said large cross-section portions to the cross-section of said intermediate portion being larger than thirty to one, a pulverulent inorganic fusible substance surrounding said intermediate portion for cooling the are products evolved upon vaporization of said intermediate portion, a pair of insulating plates of slightly smaller widths than the diameter of said tubular casing for sandwiching said link therebetween and separating said link from said substance, said insulating plates defining two pairs of ports each adapted to vent metal vapors formed at said pair of spaced necks in opposite directions into said substance, each of said pair of necks being associated with one of said pairs of ports.

' 23. In a current-limiting fus the combination of a tubular casing, a pair of plug-shaped terminal elements inserted into the ends of said oasing, the sides of said elements facing each other being provided with axial projections forming integral parts of said elements, slots in said projections for receiving a strip-shaped fuse link having a zone of restricted cross-section, a filler of quartz sand within said casing and surrounding said restricted cross-section zone of said link, a pair of insulating plates for sandwiching said restricted cross-section zone there-between, said insulating plates being supported by said projections and defining at least one port of relatively restricted cross-section for venting into said quartz sand filler the arc products formed at said restricted cross-section zone upon vaporization thereof.

24. In a current-limiting fuse the combination of a tubular casing, a pair of plug-shaped terminal elements inserted into the ends or said casing projections forming integral parts of said elements arranged on the sides thereof facing each other, slots in said projections adapted to receive the ends of a flat strip-shaped fuse link, a fiat strip-shaped fuse link inserted into said slots, said fuse link having portions of relatively large cross-section adjacent said projections and an intermediate portion of restricted cross-section, the ratio of the cross-section of said large crosssection portions to the cross-section of said inter! mediate portion being larger than thirty to one, a pair of plates of inorganic insulating material for sandwiching said restricted cross-section portion of said fuse link therebetween, a pair of plates of an organic insulating material for sand- Wiching said inorganic insulating material plates therebetween, said organic insulating material plates being supported by said projections and subdividing said casing in a direction longitudinally thereof into two substantially separate chambers, a filler of quartz sand in each said chambers, said inorganic and said organic material insulating plates defining ports arranged in registry for discharging into both said chambers the products of arcing formed at said intermediate portion of said link upon vaporization thereof.

25. In a current-limiting fuse the combination of a tubular casing, a pair of plug-shaped terminal elements inserted into the ends of said casing, projections forming integral parts of said elements arranged on the sides thereof facing each other, a metallic transversal partition within and transversely subdividing said casing into a pair of chambers, projections arranged on opposite sides of said partition and forming integral parts thereof, a pair of fuse links each interconnecting i one of said projections on one of said terminal elements and one of said projections on one of said partitions, each of said pair of fuse links having portions of relatively large cross-section arranged immediately adjacent to said projections and an intermediate portion of restricted cross-section, a pulverulent inorganic substance within each of said pair of chambers and surrounding the intermediate portion of each of said pair of fuse links, two pairs of insulating plates for sandwiching each of said pairs of fuse links therebetween, each of said pairs of insulating plates subdividing one of said pair of chambers in a direction longitudinally thereof into a pair of substantially separate subchambers, and each of said pair of insulating plates defining relatively restricted ports adapted to discharge in substantially opposite directions the products of arcing formed at the intermediate portion of each of said pair of fuse links.

26. In a current-limiting fuse the combination of a tubular casing, terminal elements arranged at the ends of said casing, a transversal partition within and subdividing said easing into a pair of chambers, conductively interconnected terminal elements arranged on opposite sides of said partition a pair of fuse links each interconnecting one of said terminal elements arranged at the ends of said casing with one of said terminal elements arranged on opposite sides of said partition, each of said pair of fuse fuse links, two pairs of insulatingplates' each for sandwiching' one of saidpairs' of fu'se 'links therebetween, each of said pairs" of insulating plates subdividing one: of said pair of chambers in' adirection longitudinally thereof into a pair of substantially separate 'sub'chambers, each of said pair of insulating" plates definingrelatively restricted ports adapted to discharge in sub stantially opp'osit'edirections into said subchambers the products of arcing" for-med at the' inter mediate portion of each of said pair of fuselinks' upon vaporizationthereof;

27. In a current-limitingfuse"th'e 'comblnat'iorrof a tubular-casing; an insulating mandrel arranged within said casing in" coaxial relation thereto, a cylindrical fuselink fitted on said mandrel, said fuse link having a plurality'of periph= eral slots situatedwithin the same transversal plane and resulting in the formation of a restricted cross-section portion consisting of a plu= rality of necks defining parallel current paths,- the ratio of the cross-section of the nonslotted portion of said fuse link-to the total cross-secti'on of said plurality of necks'being'largerthan thirty to one, a tubular insulating member fitted onsaid link anddefining a plural-ity of substantially radial orifices; oneassociatedwith each of saidplurality of necks,-for permitting the escape of metal vapors evolved upon vaporization of said pluralityof necks; said orifice's'extending in gen-- erally" radial direction, anda pulverulent inorganic cooling substance within said casing on" the outside of said insulating member for re-' ceiving said metal vapors escapingfrom said plurality of orifices.

28. In a current-limitingfusethe combinationof' a tubular casing, an insulating mandrel ar ranged within said casing in coaxial relation thereto, a cylindri'cal fuselink-fitted on said man"- drel,- saidfuse link having at least one peripheral slot resulting in theformation of a restricted cross-section portion; a tubular insulating mem ber fitted on said link and defining at leastone orifice for permitting" the escape of metal va-- por's-formed upon-vaporization of said restricted cross-section portion; said or ifice extending in' a direction generally radially of said casing, man

drel, link and insulating" member; and a pul verulentsubstan'ce within saidcasing'on the outside of said insulating member for receivingsaid metal vapors from said orifice.

29. In a current limitingl fuse the combination of a fuse link having relatively large cross-section portions and an intermediateportion' of restricted cross-section, the ratio of the cross-section of said relatively large cross-section portions tothe cross-section of said intermediate cross-section portion'b'ein'g at least thirty to one, solid insulating means in" immediate contact with and enclosing said link and defining an orifice substantially in registry with said intermediate portion for the escape of metal vapors formed upon fusion thereof, a-bodyof'qua'r'tz sand at the downstream end of said orificefor'receiving the metal vapors escaping therefrom, andvalve means for'substantially closing said orifice un til after the arc-generated pressure at=the up- 18 streamend of said orifice exceeds a" predeterruined pressure level.

30. Acur'rent-llmiting fuse comprising a-tubularcasing, terminal elements at-the ends of said casing; a 'fuse link within said casing conductivelyi inter-connecting said terminal elements, said quenching fillerlink" Within link having relatively longand wide axially outer portions and an extremely short and narrow axially inner portion, the ratio of the cross secti'on ofsaid axially outer portions to the cross seetion' of sai'd axially inner portion being atleast thirty to one, and a pulverulent fulgurite form mg arc quenching filler within said casing for deener'gizing the gasesformed by vaporization of said axially inner'po'rtion of said link.

31-. A-current-limiting fuse comprising a tubul'a'r casing; terminal elements at the ends of said casing; a fuse link within said casing conductively interconnecting said terminal element's;-

saidJ ink h'a-Vingrelatively long and Wide axially outer portions and an extremely short and nar row axially inner porti'on, said axially outer p'or'-- tions approximating the critical width beyond which further increase in width has no substantial effect upon the current-carrying capacity ofsaid link, the ratio of the cross-section of said axially outer portions to the cross-section of said axially inner portion being at least thirty to one; and a pulverulent fulgurite-forming are within said casing for deeper-- gizin-gl the gases formed upon vaporization of said axiallyinner portion of said link.

32; A current-limiting fuse comprising a tuula r casing,- terminal elements at the endsof said casing, a fuse link conductively interconnecting-said terminalelements, said fuse link withinsaid casing-having relatively long and wide axially outer portions and an extremely short and narrcwaxially inner portion forming a plurality of current'paths in parallel, the ratio of thecross-section of said axially outer portionsto" the cross-section of said axially inner porti'on'being at least thirty to one, and a body of qu'a rti sand within said casing for cooling the gases formed: upon vaporization of said axially inner portion of said link.

33'. A curi'ent limitin'g fusecomprising atubula'r casing, said casing;- an extremely thin metal foil fuse said casing conductively interconnectlng" said terminal elements, said link having relatively long and wide axially outer portions and an-extremelyshort and narrow axially" inner portion, said axially outer portions approximating the'critical width beyond which further incr'ease inwidth has no substantial effect upon the current-carrying capacity, the ra-- said casing, an extremely thin metal foil fuse" link w-ithin' said casing conductively intercon nectin'g said terminal elements, said link having relatively long and wide axially outer portions and xa'd'extretxiely short and narrow axially inner terminal elements at the ends of portion forming a pair of current paths in parallel, said axially outer portions approximating the criticalwidth beyond which further increase in width has no substantial effect upon currentcarrying capacity, the ratio of the cross-section of said axially outer portions to the cross-section of said axially inner portion exceeding thirty to one, and insulating support for said link arranged diametrically within said casing and extending in a direction longitudinally of said link, and a quartz sand filler within said casing to both sides of said link and said support.

35. A current-limiting fuse comprising a tubular casing, terminal elements at the ends of said casing, an extremely thin metal foil fuse link within said casing conductively interconnecting said terminal elements, said link having relatively long and wide axially outer portions and an extremely short and narrow axially inner portion, said axially outer portions approximating the critical width beyond which further increase in width has no substantial eifect upon current-carrying capacity, an insulating support for said link supported by said terminal elements and arranged diametrically within said casing, said support consisting of two portions sandwiching said link therebetween, a quartz sand filler to both sides of said link and said Support, and a pair of coaxial venting passages in said support transverse to the general direction of current flow in said link for venting products of arcing formed upon vaporization of said axially inner portion of said link to opposite sides of said link and said support into said quartz sand filler.

36. A current-limiting fuse comprising a tu bular casing, terminal elements at the ends of said casing, an extremely thin metal foil fuse link within said casing conductively interconnecting said terminal elements, said link having relatively long and wide axially outer porticns and an extremely short and narrow axially inner portion, the ratio of the cross-section of said axially outer portions to the cross-section of said axially inner portion being at least thirty to one, an insulating support for said link supported by said terminal elements and arranged diametrically within said casing, said support consisting of two portions sandwiching said link therebetween, a quartz filler to both sides of said link and said support, and a pair of coaxial venting passages in said support transverse to the general direction of current flow in said link for venting products of arcing formed upon vaporization of said axially inner portion of said link to opposite sides of said link and said support into said quartz sand filler.

37. A current-limiting fuse comprising a tubular casing, terminal elements at the end of said casing, a ribbon-type fuse link within said casing interconnecting said terminal elements, said link having relatively long and wide axially outer portions and a relatively short and narrow axially inner portion, an insulating body of considerabie mechanical strength in engagement with and at least in part surrounding said axially inner portion of said link to preclude substantial escape of products of arcing from the point where said axially inner portion of said link is located, a pulverulent arc-quenching filler within said casing surrounding the outer surface of said insulating body, a passage in said insulating body between said point where said axially inner portion of said link is located and said pulverulent arc-quenching filler, and pressure responsive'sealing means for normally clos'-' ing said passage and for causing a sudden breakdown of pressure at said point where said axially inner portion of said link is located upon occurrence of a predetermined pressure at said point.

38. A current-limiting fuse comprising a tubular casing, terminal elements at the ends of said casing, a ribbon-type fuse link within said casing interconnecting said terminal elements, said link having relatively long and wide axially outer portions and a relatively short and narrow axially inner portion, an insulating body in engagement with and substantially surrounding said axially inner portion of said link to normally preclude substantial escape of products of arcing from the point where said axially inner portion of said link is located, a pulverulent arc-quenching filler within said casing surrounding the outer surface of said insulating body, a pair of coaxial passages in said insulating body extending transversely to the general direction of current flow, through said link and connecting said point where said axially inner portion of said link is located with the space occupied by said pulverulent arc-quenching filler, and a pair of pressure responsive sealing means for normally closing said pair of passages and for causing a sudden breakdown of pressure at said point where said axially inner portion of said link is located at the occurrence of a predetermined pressure at said point.

39. A current-limiting fuse comprising a tubular casing, terminal elements at the ends of said casing, a, ribbon-type fuse link within said casing interconnecting said terminal elements, said link having relatively long and wide axially outer portions and an extremely short and narrow axially inner portion, the ratio of the crosssection of said axially outer portions to the crosssection' of said axially inner portion being at least thirty to one, an insulating support for said link arranged diametrically within said casing and supported by said terminal elements, said support consisting of two fiat plates sandwiching said link therebetween, a pair of coaxial venting passages in said plates arranged transversely to the general direction of current flow in said link for venting products of arcing formed upon vaporization of said axially inner portion of said link to opposite sides of said link and said support, a quartz sand filler within said casing arranged to opposite sides of said link and said support, and a pair of pressure responsive sealing means for normally closing said pair of venting passages and for causing a sudden breakdown of pressure at the point where said axial-1y inner portion of said link is located upon occurrence of a predetermined pressure at said point.

FREDERICK J. KOZACKA.

References Cited in the file of this patent UNITED STATES PATENTS 2 Number Name Date 569,373 Downes Oct. 13, 1896 768,487 Simons Aug. 23, 1904 928,998 Murray July 27, 1909 1,124,133 Hershey Jan. '5, 1915 1,265,223 Marcus May 7, 1918 1,852,104 Bowie Apr. 5, 1932 1,857,098 Glowacki May 3, 1932 2,028,721 La Mar Jan. 21, 1936 2,181,825 Wood Nov. 28, 1939 2,189,101 Carter Feb. 6, 1940 

